Remove use of utf8_encode (deprecated in PHP 8.2)

classes/local/store/s3/client.php

@@ -667,7 +667,13 @@ private function get_nice_filename($headers) {
 
  if (!empty($originalfilename)) {
  $result['Content-Disposition'] = $contentdisposition;
- $result['filename'] = 'filename="' . utf8_encode($originalfilename) . '"';
+ // The filename parameter must be in ISO-8859-1, however it works in browsers if
+ // you treat the original UTF-8 string as ISO-8859-1 characters. To achieve that
+ // here, we encode the UTF-8 as if it were ISO-8859-1. This behaviour is hideous
+ // so it would be nice to use the optional filename* field (RFC 5987) but S3 still
+ // complains if we do that.
+ $jankyfilename = \core_text::convert($originalfilename, 'ISO-8859-1');
+ $result['filename'] = 'filename="' . $jankyfilename . '"';
  $result['Content-Type'] = $originalcontenttype;
  }
  }